The proposed research project is designed to test the hypothesis that the normal elevation of mean arterial blood pressure with increasing age is due to an increase reaction of resistance vessels to norepinephrine. It is further designed to reveal the underlying mechanisms for possible increased reactivity. Other studies concerned with age-related changes in vascular contraction have used rat aortic tissue, where there is significant hypertrophy with age. The effect of such hypertrophy is, predictably an increased reactivity to contractile agents, but such evidence cannot be used to explain hypertension development with age. Since blood pressure is partially a function of peripheral resistance, the resistance segments of the vascular tree, rather than the aorta, require investigation. Strips of vascular smooth muscle will be taken from small diameter arteries of cutaneous, mesenteric, and skeletal muscle vascular beds of rabbits aged two months to four years. The strips will be bathed in various solutions of norepinephrine, potassium, isoproterenol, and blocking agents, to determine dose response curves of vessels with age. From this data, and additional physical measurements, the relative contributions of vessel hypertrophy and adrenergic receptor sensitivity to change in vessel reactivity will be assessed.